What's the extent of the universe!

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Hi all! I devote lot of my time in just watching the sky at night.Well its beautiful.Well the stars they are so tiny when I look them from my hostel's roof. But actually they are the power houses of this universe. We all know about the big bang how our universe came out suddenly out of nothing and I learned from discovery that its still expanding. Expanding?? Well expanding thats a confusing term for me .If there was no universe before Big Bang then what was there and now even if universe is expanding then what is its limits? Is it some kind of boundary or what? Well I hope some the geeks here will be able to answer me this mystry for me....